I've gotten plenty of use out of my Locusts and Stingers, an Urbanmech, a pair of Hermes IIs, and even a Vulcan in my collection. About the only 'Mech in my moderately sizable collection that I've probably never actually fielded in a battle is an Assassin, and only because I've got several other light and medium 'Mechs that perform basically the same role. I've got nothing against using it, and I know it can be fairly effective at harassing vehicles and light "bug" 'Mechs, but there really hasn't been any essential need to field it either.
A basic Locust 1V makes a decent scout and harassment unit for a bargain price, has sufficient armor to withstand a bit more punishment than its slower 20T companions, and will easily go through terrain that no hovercraft can; a hovercraft is a VERY viable replacement in SOME situations, not others. The Urbanmech is a defensive specialist in either broken or urban (as the name implies) terrain, but is at a severe disadvantage against heavier units with long lines of sight, or where maneuverability is required. The Hermes II makes a decent scout hunter and highly flexible command unit for a "bug" lance, and the 2M variant can be a monstrous back-stabber. The Clint, while similar to the basic HER-2S, is disturbingly under-armored for the tonnage, sacrificing armor for jump capability, but sufficient against some lighter units. The Vulcan has the speed and long-range sniper weapon to "park" vehicles that would otherwise be a serious threat to your other units, not the kind of unit you'd want to field in quantity, but one in a force is potentially a critical asset in various situations. Then there are units like the Whitworth, which are impressively effective for their C-Bill cost (a nearly optimal LRM and ML bracket firer), but just not heavy enough to engage most "line" 'Mechs one-on-one; I've run them in numerous campaigns and had VERY good luck with them in a supporting role, and like a Catapult, often successfully engaging badly damaged heavies with the lasers after the LRMs run dry.